The new 4K Sony PXW-FS5 has been extremely popular with small production companies, one-man band shooters as well as those who already own a PXW-FS7 and even F5/F55 cameras as for them it is the perfect B-cam.
However, the camera, just as any other camera, comes with its own quirks. The FS5 records internally in UHD (3840 x 2160) up to 30p in the XAVC-L (Long GOP) codec, which is a more efficient 8bit 4:2:0 codec compared to the Intra frame XAVC-I codec where ever frame is compressed at once.
This however comes at the expense of larger files and faster media such as XQD required to sustain the data rate. Sure a 10bit 4:2:2 file will technically give you more information to play with in post-production when it comes to grading or VFX, but the XAVC-L codec is so much more efficient, allowing for the use of more affordable SDXC cards and saving quite a bit of disk space while still preserving a lovely image quality.
Upon release some users reported that the FS5 would not allow simultaneous recording in 4K UHD internally and outputing the same via HDMI. Not only that but even if you have an Atomos Shogun attached just for monitoring purposes, as soon as you press record in the camera, the video signal out of the outputs would shut off and you'd see a black screen on the Shogun's display.
This is far from perfect, and Sony quickly reacted by releasing a remedial firmware update 1.10, which allows users to record externally in a high quality 10bit ProRes codec for example on an Odyssey7Q+, PIX-E5 or Atomos Shogun/Assassin as well as have the internal XAVC-L recording in the camera as a backup. The caveat with this is that you'd loose the ability to have the LCD and viewfinder as all the outputs are connected.
Therefore, if one wants to still retain the LCD and EVF, while recording externally in UHD, then internal recording is not possible. Maybe this is something that would be possible in the future, but as of now, possibly due to internal limitations - having an EVF, LCD output, recording internally UHD and externally at the same time is not possible.
However, you can recorder Internally & Externally in UHD now with firmware 1.10 (but without the LCD/EVF, but you can still use the amazing display of any external 4K recorder from Atomos, Convergent Design, or Video Devices).
This from Sony:
Firmware Version 1.1
Version 1.1 allows simultaneous QFHD internal recording and QFHD output to an external unit that serves as the monitoring device. A new menu item (“QFHD REC OUTPUT”) in the recording menu permits switching between simultaneous QFHD internal/external recording, with loss of VF and LCD operation and internal recording. The loss of VF/LCD does not affect HD operations.
Version 1.1 is merely the next step in the PXW-FS5’s ongoing development. A future firmware build will add “FS RAW” recording. The OLED and VF will remain active regardless of the choice of format.